Heaters are described in U.S. Pat. Nos. 4,570,715, 4,572,299, 4,616,705, and 4,704,514 which utilize copper heating cores. These heaters are useful for heating long intervals of subterranean formations. Heating subterranean formations can be useful to mobilize oils or pyrolize oil shales.
Copper is not a typical material for a heater core. Typically, a material with a higher resistance would be chosen but subterranean heaters can have heating elements of up to 1,000 feet long and elements with higher resistances would require excessively high voltages. An element with a low electrical resistance is therefore needed for these subterranean heaters. Copper-cored heaters fulfill the low resistance requirements, but have a shortcoming in that they have a very temperature-dependent resistance. The resistivity of copper changes by a factor of about four between 35.degree. C. and a typical core operating temperature of about 800.degree. C. When the cores are cold the heat that can be generated is limited by the current which can be applied. Heater control by maintaining a constant current is therefore preferred when the heater is warming up. After the heater is hot, constant current control becomes undesirable. Due to the heater's core electrical resistance increasing with temperature, a core which is too hot will generate even more heat if a constant amount of current is passed through it. Maintaining constant power results in the current being reduced if the electrical resistance of the core increases due to an increase in the core temperature.
Control of heaters which are providing heat to subterranean formations is further complicated by the large number of such heaters within a viable commercial application. Manually adjusting the voltages being applied to the heater cores to stay within current, power, and temperature constraints is a time consuming and labor intensive effort. Heating up periods are extended by failure to maintain maximum current and/or power.
It is therefore an object of the present invention to provide a method to control a heater which maintains a set point of either current, or power, depending on which variable is limiting the heater operation, and which automatically switches from one mode of control to the other as required in order to maintain a maximum heat output rate. It is a further object to provide such a controller which provides a smooth transfer from current control to power control when the power generated has increased to the power set point. It is another object to provide such a controller in which power supplied to the heaters automatically reverts to current control after power interruptions, followed by an automatic switch to power control once the power setting is reached.